1. Why do we itch? Why do we itch?
WE ALL KNOW HOW AN itch feels, but there’s still a lot we don’t know about how it works. An observation made 2,000 years ago by the Buddhist philosopher Nagarjuna suggests the complexity of the sensation: “There is pleasure when an itch is scratched. But to be without an itch is more pleasurable still.”



There are many serious diseases in which itch is constant, extremely unpleasant, and very difficult to treat. Fortunately, most of us only experience the transitory itch from an insect bite or a wool sweater, but even those relatively trivial itches are tough to describe.

For example, the feeling of pain and itch are similar. They’re both irritating and you want them to go away, but each has a unique quality.



Did You Know … That famous portrait of Napoleon Bonaparte with one hand in his waistcoat and the other behind his back? Some scientists suspect that his hands are hidden because he’s scratching himself. That’s the problem with standing there forever as the painter paints: you just get itchy. But many believe Napoleon did have some sort of irritating skin condition. For a long time scabies, a disease caused by tiny mites, was thought to be the cause, but most medical experts doubt that now, and the cause of Napoleon’s itch is still unknown.

For some time scientists believed that pain and itch were dependent on the same nerve pathways, pain simply being an amplified version of itch. The fact that scratching actually causes pain—enough pain to subdue the itch at least temporarily—can be interpreted as amping up the stimulation enough to cross the threshold between itch and pain. Some compounds that relieve pain, like opioids, can at the same time cause itchiness, as if the threshold were crossed in the other direction. And those rare and unfortunate individuals who are insensitive to pain, a life-threatening condition, also do not experience itch. However, the belief that pain and itch share the same neural infrastructure has been largely abandoned. The itch network has its own specific nerves and transmitter molecules, even though there is some interplay between those and their counterparts for pain.



Of course the two are very different. Itch is confined to the skin, whereas pain can occur virtually anywhere in the body. And our response to pain is withdrawal—snatching your hand away from the hot stove—but we react to an itch by attacking it, usually with our fingernails. Both reactions make sense, especially if you think that part of the reason we scratch is to remove anything, like a biting insect, that is responsible for the itch.

Interestingly, if we don’t scratch hard enough to cause pain, scratching an itch can, as Nagarjuna claimed, bring pleasure.

It makes sense to think of an itch as a local event on the skin, but that’s not true. If you were to track the itchiness of, say, a mosquito bite, it starts at the site of the bite, where the body recognizes the chemicals released by the mosquito as foreign and reacts by releasing histamine, a notorious itch promoter. Histamine causes local nerves to fire, and those signals travel to the spinal cord, then the brain, where the impulses of those nerves are registered as an itch. So that itch on your arm is really in your brain, a very hard place to scratch.



(I think the idea that itch is centered in the brain explains something I’ve noticed: if I’ve been out in the woods and have collected several mosquito bites, even if most of them are quiescent, scratching the one that’s itchy makes all of them start to itch. Again, the explanation lies in the brain, not in the individual bites.)



Did You Know … If you are itchy, there are several things you can do. Over-the-counter anti-itch remedies can help, but there are a couple of remedies that come from the scientific literature, too. For instance, seventy years ago scientists noted that light pinpricks around the site of an itch eliminated it for up to forty-five seconds. This couldn’t have been a substitution of pain for itch because the mild pain of the pinprick disappeared thirty seconds before the itch returned. Others have noticed that pressing on the skin around the itch site, rather than direct scratching, also dampens the itch sensation.

More evidence that the itch is in your mind comes from experiments showing that itch can be contagious (although apparently pain—also in your brain—isn’t contagious: just another difference between the two). When volunteers were shown videos of other people scratching or images of insects on their skin, they scratched themselves much more than they did when watching a neutral video. Interestingly they didn’t necessarily scratch the same place on their bodies as the individuals in the video did; most of the time they just scratched their heads. But it was a real effect and raises the question of why this should happen. It resembles contagious yawning, where even the word “yawn” can make people do it. There is apparently no evidence—yet—that just reading about itchiness can make you itchy. Is there?



TRY THIS AT HOME! A scientist named Theodore Cornbleet studied a group of itchy volunteers and found that whenever they scratched, the length of the scratch varied depending on the location. Itchiness on the tips of the fingers provoked very short scratches, roughly 2 millimeters (about one-sixteenth of an inch) long, but itchy spots on the back were attacked with mega-scratches stretching 80 millimeters (more than three inches). Cornbleet argued that this happened because touch sensitivity varies all over the body, and where the sensitivity is less, as on the back, longer scratches are needed to influence enough neural receptors to affect the itch.

You can test your own touch sensitivity: Bend a paper clip into a U shape, then test (without looking) how close together the two ends can be before you can’t tell them apart. Do that for your fingertips as well as your back, your calf, the bottom of your foot. Each will be different.